Out of the multitude of available options, I had an initial play with the following: FriendFeed, Mugshot, SecondBrain, Youmeo, and SocialURL.
Of these, FriendFeed was really the only one to come close to what I was looking for, delivering a seamless RSS feed from 7 of the sites I use. Normally that would only be useful for someone-not-me, since I'm the one putting the content there in the first place, so an update to the feed shouldn't really surprise me. However, FriendFeed allows you to define "imaginary" friends, by specifying their public pages (e.g. blogs, Flickr, LibraryThing) and it turns that all into one big combined feed. To be honest, that's not immediately useful, since I already subscribe to a host of individual feeds giving me the same info, but if my PC dies or I'm away from home it would make getting updates much easier.
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